The present invention concerns a method for controlling the profile of a non-woven fleece.
The present invention also concerns an installation for production of a fleece of non-woven fibres.
It is known to produce a fibrous product, such as a fleece web, in a carding machine or other apparatus such as, for example, a pneumatic fleecing machine. The fibre web thus obtained is fed into a crosslapper in which the web is alternately folded one way and the other on a delivery belt. The fleece is thus made up of web segments inclined alternately one way and the other, which overlap. The folds between successive segments are aligned along the lateral edges of the fleece produced.
The fleece of fibres which is obtained is generally intended for subsequent consolidation processing, for example by needling, by coating and/or etc. to obtain a consolidated non-woven product.
FR-A-2 234 395 teaches the speed ratios that must be complied with in the crosslapper to control the thickness of the fleece at any point across its width.
DE-C-1 287 980 teaches placing directly at the exit of the crosslapper, above the longitudinal axis of the fleece, a gauge 32 which detects defects in thickness/surface weight of the fleece along its axis. This detection is received by a processing device which, in case of a discrepancy from a set reference, corrects in particular the speed of the delivery belt of the crosslapper when an incorrect overlapping of the web segments forming the fleece produces transverse wads or, conversely, gaps in the form of transverse grooves in the fleece. In case of a deviation of the thickness of the fleece relative to a set reference, the processing device orders a corresponding variation in the rotation speed of the doffer of the carding machine which is installed upstream of the crosslapper.
EP-A-0 315 930 proposes a crosslapper producing a fleece having, in cross-section, a non-uniform thickness/surface weight profile. To this end, the lapper carriage, which deposits the web at a variable point on the width of the delivery belt, is actuated at a speed which varies in relation to the speed of the belts which eject the web across this carriage to deposit it on the delivery belt of the crosslapper. If, at a given position on the fleece width, the carriage moves at a speed higher than that at which it feeds the fibre web, the fibre web is stretched, and this reduces the thickness of the fleece at that place. If, on the other hand, the carriage speed is below the feeding speed, the fibre web is deposited in a compressed form, which increases the thickness of the fleece at that point.
EP-B-O 371 948 describes a method intended to pre-compensate for the defects occurring during subsequent consolidation, in particular needling, by locally varying the thickness of the web introduced into the crosslapper. This is achieved by automatically controlling the speed of a doffer of the carding machine relative to the speed of the carding drum. The more quickly the doffer turns relative to the drum, the more the surface weight of the fibre web formed by the doffer is reduced.
FR-A-2 770 855 describes various improvements to this process and proposes modified embodiments combining modulation of the longitudinal profile of the fibre web produced by the carding machine or like apparatus with a stretching and/or compression action of the fibre web as it leaves the lapper carriage of the crosslapper.
A computer-controlled processing device enables the user to input a reference profile desired for the fleece and then commands the fibre web production equipment and/or the crosslapper in a manner which is calculated in view of realization of the requested profile. In practice, the user of the installation attaches decisive importance to the profile of the consolidated fleece obtained. This profile is inevitably modified by functional imperfections of the crosslapper and of the consolidation machine, notably when the latter is a needling loom. Needling looms perform the function of interlacing the fibres. At the same time they have the disadvantage of reducing the transverse dimension of the fleece and providing a fleece that is thicker along the edges than in the median zone.
The installations described in EP-B-O 371 948 and FR-A-2 770 855, as well as the crosslapper described in EP-A-0 315 930, in principle make it possible to give the fleece leaving the crosslapper a non-uniform profile which pre-compensates for the defects which will be produced by the needling loom. But in practice, perfect pre-compensation is very difficult to achieve, necessitating laborious adjustments. Moreover, it is not certain that a good initial adjustment will suffice in the long term to obtain a consolidated product in conformity with the expected ideal profile.
The object of this invention is thus to propose a method and a production installation which makes it possible for the user to obtain, more simply and more reliably, the profile desired for the consolidated fleece.
According to a first aspect of the invention, the method for controlling the transverse profile of a non-woven fleece in an installation for production of said fleece, in which, at a measuring station, a physical magnitude of the fleece is detected and on the basis of this detected value the profile of the fleece is corrected by adjusting an operation parameter of at least one fibre-arranging unit situated in the installation upstream of the measuring station, is characterized in that
at the measuring station the physical magnitude is detected at several points across the fleece width so as to record a transverse profile of the fleece;
in case of a discrepancy between the profile recorded and a reference profile, the operation parameter is corrected when said unit is working the fibres which will be located at the point on the fleece width where the profile discrepancy appeared.
xe2x80x9cFibre-arranging unitxe2x80x9d is the term applied to a unit belonging, for example, to a carding machine or a crosslapper and which has an effect on the arrangement or distribution of the fibres in the fibre web or the fleece, and which has an influence in particular on the surface weight of a xe2x80x9cfibre web cross-sectionxe2x80x9d or of a point on the fleece width. xe2x80x9cFibre web cross-sectionxe2x80x9d is the term applied to a transverse section of a fibre web or other fibrous product at a specific point along its length. This cross-section is characterised in particular by its surface weight, which can vary from one cross-section to another.
With the invention, the transverse profile obtained is continually or intermittently checked, and targeted corrections are carried out if there is a discrepancy between a point on the profile obtained and the corresponding point on the reference profile.
The correction can be carried out using a method known per se according to EP-A-O 315 930, EP-B-0 371 948 or FR-A-2 770 855.
The physical magnitude which is measured can be chosen from a wide range. For example, the permeability of the fleece to a given radiation can be measured. This permeability constitutes a physical magnitude representative of the local surface weight.
The method described in FR-A-2 770 855 requires precise knowledge of the xe2x80x9cdelay lengthxe2x80x9d, i.e. the length of fibre web between a first fibre web cross-section in the process of being deposited on the fleece being formed in the crosslapper, and a second fibre web cross-section located at the point on the fibres path, where adjustment of the specific weight is carried out upstream of the crosslapper, in particular in the carding machine. If the fibre web is subjected to stretching or compression between these two cross-sections, a correspondingly corrected delay length must be taken into account. The corrected delay length corresponds to the total running length travelled by the lapper carriage above the delivery belt between the time-point when it deposits the first cross-section mentioned and the time-point when it deposits the second cross-section mentioned. By knowing this delay length, possibly corrected, one knows the point at which a fibre web cross-section undergoing thickness/surface weight correction in the carding machine will be deposited in the fleece width.
The delay length, possibly corrected, can be theoretically determined at any time in a given production installation, programmed in a given way. In practice such a theoretical determination can be difficult to implement and may not produce a perfect result. It is especially difficult to take into account certain elements such as the elasticity of the fibres, which risk being stretched or, conversely, becoming recompressed at certain points in their path.
According to an aspect of the invention which can, independently of profile control, provide a useful addition to FR-A-2 770 855, the delay length is determined experimentally, or at the very least the theoretically determined value is experimentally finalized. For this purpose, an initialization step is implemented with the help of a particular feature of the fibrous product, whose longitudinal position is determined along the fibrous product when travelling through said fibre-arranging unit, and whose transverse position is then determined in the fleece produced. Thanks to this more precise knowledge of the delay length, the profile-control method according to the invention can be more effectively implemented.
Advantageously this particular feature is a pseudo-defect generated by the arranging unit. It is also advantageous for the particular feature, notably the pseudo-defect, to be detected by the means for detecting the physical magnitude. In view of this, the initialization process forms a still more advantageous combination with the profile-control method per se.
Knowing the length of fibre web necessary to feed the lapper carriage during a reciprocation of the latter, it is possible to deduce therefrom all the successive fibre web cross-sections that will correspond to the same transverse position on the fleece. And it will be possible to deduce therefrom the position of the fibre web cross-sections corresponding to any other transverse position on the fleece.
In practice, at the same time that the passage of the particular feature is determined at its passage across the arranging unit, the position of the lapper carriage in its reciprocatory cycle can be determined. Then, each time the lapper carriage passes this position in its cycle again, it will be known that the fibre web cross-section being worked by the arranging unit is intended to settle in the aforesaid transverse position of the fleece.
In an improved version, the initialization can advantageously comprise a step consisting of phase-shifting the successive particular features in relation to the reciprocatory cycles of the lapper carriage, until these successive particular features are located in a particular position on the fleece width, and notably on the central axis of the fleece.
The fibre web cross-sections which will be located on the central axis of the fleece have thus been determined.
By simple subdivision of the length of the fibre web between two such successive sections, fibre web cross-sections are found which will be located at different points across the fleece width. This subdivision can be carried out in a deliberately irregular manner, to take account, for example of non-constant stretching/compression on leaving the lapper carriage.
Once initialization is completed, the production step begins, and there is then no longer any need to provide the fibre web with xe2x80x9cparticular featuresxe2x80x9d or xe2x80x9cpseudo-defectsxe2x80x9d. When a correction is to be made to the transverse profile of the fleece, this correction is made on the fibre web, in a fibre web cross-section chosen by reference to the fibre web cross-sections which are known to be located along the fleece axis.
The correction can also be carried out by modifying the speed ratio between the lapper carriage (then becoming the fibre-arranging unit, serving to apply the correction) and the speed of movement of the fibre web across the lapper carriage when the lapper carriage is located above the point on the fleece width where the profile discrepancy was found. This method has the advantage of necessitating no determination between the longitudinal positions on the fibre wed and the transverse positions on the fleece, but can have the disadvantages referred to in FR-A-2 770 855 in connection with EP-A-0 315 930, in particular relatively poor effectiveness of correction when using relatively elastic fibres.
It is therefore preferred according to the invention to make surface-weight corrections during production of the fibre web upstream of the crosslapper. On the other hand, it is advantageous to correct the width of the fleece obtained when it deviates from the reference width, by adjusting the end-points of the pathxe2x80x94or strokexe2x80x94travelled by the lapper carriage of the crosslapper.
In this case also, the lapper carriage is a fibre-arranging unit which can be acted upon to apply a correction to the profile within the scope of the control method.
The corrections made can have the effect of modifying the position of the lapper carriage for which the fibre web cross-section intended to be located on the fleece axis travels through the fibre-arrangement unit. The said position of the lapper carriage can then be recalculated by applying to the position previously known a variation theoretically calculated from the foreseeable effects of the correction.
According to another aspect of the invention, the installation for production of a non-woven fibre fleece, comprising fibre-arranging units, detection means for measuring a physical magnitude of the fleece moving at a measuring station, and control means receiving a signal provided by the detection means and delivering to at least one of the arranging units, a modified control signal in case of a discrepancy between the physical magnitude read and a reference value, is characterized in that:
the detection means are designed to measure the physical magnitude at different points across the fleece width;
the control means compare the physical magnitude of each point with a reference value related to this point, and in case of a discrepancy at a point, apply a corrected command when the arranging unit is working fibres intended to be located at said point.
Further features and advantages of the invention will become clear from the following description, which relates to non-restrictive examples.